Summary: Information regarding the minimal requirements for activation of primed T cells is readily available. T cell clones and T cell hybridomas can be activated by anti-CD3 antibody or purified antigen-MHC proteins. Activation of these primed T cell lines can occur in the absence of specialized presenting cells and their accessory molecules. This project is to define the minimal requirements for activation of naive T cells and to evaluate the effects of additional stimuli on the magnitude and type of T cell activation elicited. With the use of a potent peptide-MHC complex, we have been successful in the activation of phenotypically naive CD8 positive transgenic T-cells. These T-cells have been stimulated to proliferate, secrete IL-2 and mature to cytolytic effectors in the absence of additional signals from antigen presenting cells. Through the use of immobilized peptide-MHC complexes and purified recombinant soluble ICAM-1, we have evaluated the effects on T-cell activation of adding an isolated costimulatory signal. ICAM-1 can increase proliferation and cytokine production of CD8+ T-cells with varying TCR signal intensity, generated through different peptide-MHC complexes. Using anti-CD3 stimulation of CD8+ and CD4+ T cells, we have demonstrated that ICAM-1 has a differential effect on the two T-cell subsets. We have shown that ICAM-1 enhancement of CD8+ T-cell activation can occur in the absence of an organized immunological synapse. Evaluation of other purified adhesion/costimulatory molecules may reveal different effects. We have also evaluated the effect of a mucin, similar in structure to CD43, on T-cell activation and defined some of the requirements for CD43 movement away from the T-cell contact site. We are beginning to correlate some of these in vitro effects with in vivo immune function using an adoptive transfer system. Using SPR, we have probed the binding of the CD8 molecule and beta-2-microglobulin to MHC alpha 3 in order to better understand the role of this binding in T-cell activation and antigen presentation. In addition, the functional effects of other other soluble adhesion/costimulatory molecules may be correlated to binding using SPR analysis. This system will allow us to evaluate specific changes in naive T cell responses caused by the deletion or addition of lymphokines, adhesion molecules or costimulatory molecules. This can be done in the absence of unknown contributions from antigen presenting cells and correlated with in vivo data using the same transgenic T-cells used in vitro. The molecular requirements for T-cell activation and the varying phenotypes of this activation are very basic to the generation of successful immunity and the modulation of aberrant immune responses.